I was reading the book yeast by Chris White and Jamil Zainasheff. In the book, they talk about measuring how many cells are in a slurry by using a microscope or by dilution until the slurry is clear in a test tube.

However, they also mention that the size and weight (density) of a yeast cell is known, the density of water is also known. So couldn't one take a yeast slurry, measure its weight and volume and use this measure of density to determine what percentage of the mass is composed of yeast, and what percentage is composed of water?

Clearly, since wort is more dense than distilled water, you may have to wash the slurry first. But this seems much simpler than whipping out a microscope or doing multiple dilutions.

Should this work?

Note: I'm talking about measuring the number of yeast cells in a slurry, not their viability or vitality.

2 Answers
2

In theory if you can accurately measure your density and weights its possible. But in most real world applications your two measures will be contaminated by protein, wort, alcohol and hop debris. That will make an accurate measurment impossible, albeit a scientifically sound assumption.

Yeast also aren't always the same size they swell and shrink a bit as a population depending on environmental conditions and growth phase. Again these things confound and put error in the measurements of density and weight per cell.

If you can get a particularly clean sample that is just wort or water and naturally compacted yeast then this should work, at least if calibrated initially against a cell count.

The trouble is that most slurries from previous ferments contain a fair amount of trub, which can be difficult to account for. If you have a conical, then cropping from the top-middle of the cone should give you a viable as well as clean sample.

Another tricky part is that may also need to know the ratio of yeast to liquid, since densities of slurries can vary, unless you let it settle completely in a flask and then draw off all the liquid.

You don't have to wash the slurry to remove the wort if you take a sample of clean wort and measure its density. This saves a washing step which could be a pain to do thoroughly.